Smooth slow motion with optical flow in Video by Zaweba
Choose repeat frames, frame blending or optical flow for slowed clips in Video by Zaweba, and learn how each builds the frames that slow motion needs.
Slowing a clip down creates a problem. A second of footage at normal speed has a fixed number of frames, and when you stretch it to fill two seconds, there are gaps between them that need filling. How those gaps are filled decides whether the slow motion looks smooth or stutters. Video by Zaweba lets you choose the method for each clip.
Three ways to fill the gaps
The first is to repeat frames. Each original frame is shown for longer, so at half speed every frame appears twice. It is the simplest method. The picture stays perfectly sharp, but the motion steps rather than flows, especially at slow speeds.
The second is to blend frames. The in-between frames are made by mixing the frames either side. Motion looks smoother than with repeated frames, but anything moving shows a soft double image where the two frames overlap. On slow, gentle movement, blending often looks fine. On fast action, the ghosting can be obvious.
The third is optical flow. Video by Zaweba matches blocks of pixels between one frame and the next, works out where each part of the picture moved, and then warps and mixes the frames to build new ones in between. Because it follows the movement rather than just blending, it can make genuinely new frames, and slow motion can look much smoother.
Preview and export match
The method you choose is used in both the preview and the export. You see the real result while you edit, not a rough stand in, so you can judge whether optical flow is working on a shot before you commit to an export.
Choosing the method
There is no single best choice, which is why it is set per clip.
Repeat frames suit footage where crispness matters more than smooth motion, or where a slightly stepped look is wanted, a stylised effect for instance. They also suit footage with very little movement, where there is little to gain from anything cleverer.
Blend frames suit gentle movement: drifting clouds, slow water, a slow push in on a still scene. They are a safe middle ground.
Optical flow suits most real slow motion jobs, where people or objects are moving clearly through the frame. It does its best work when the movement is steady and the background is distinct from the subject.
Where optical flow struggles
Because optical flow is following blocks of pixels, some content confuses it. Things that cross in front of each other, such as a hand passing in front of a face, can leave warped edges where it is not sure which part belongs to which object. Very fast motion with blur, repeating patterns like fences, and sudden flashes are also hard. When you see wobbly edges or smears, try blend frames instead, or keep the slowdown less extreme.
Play the slowed section through and look carefully at the edges of moving objects before you settle on a method.
Getting the best slow motion
The best slow motion still starts in the camera. Footage recorded at a high frame rate has real frames to slow down, and needs no invented ones at moderate slowdowns. When that is not possible, optical flow in Video by Zaweba is the tool that makes ordinary footage usable at slower speeds.
Keep the slowdown in proportion. Half speed asks the app to make one new frame between each pair. Much slower asks for many more, and artefacts become more likely.
Every way of slowing a clip
The in-between setting works with every way of slowing a clip: a plain speed change, Speed / duration, rate stretch, Fit to fill and speed ramps. When you build a ramp that drops into slow motion, setting the clip to optical flow is usually the step that makes the slow part look right.
Slow motion is one of the most satisfying effects to get right. Choosing how the missing frames are made is the part most people never think about, and in Video by Zaweba it is one setting away.
